The present invention relates to loudspeakers in general, and more particularly to an acoustic apparatus including a loudspeaker fitted with an acoustic absorber on the rear side for absorption of unwanted rearward acoustic energy which might create undesirable resonant peaks in the rear low frequency end of the audible frequency spectrum.
The conventional method of absorbing acoustic energy emanating rearwardly from a loudspeaker involves the use of a thick layer of absorbent material such as glass wool or cotton, or a multiple layers of such material as felt. However, the prior art absorbent material has not sufficient degree of acoustic absorption capability, particularly for the energy in the lower frequency end of the audible frequency spectrum so that a large amount of such absorbent material was needed, and in order to obtain a flat frequency response characteristic an enclosure is required which is fitted inside with a layer of such absorbent material. However, the use of the enclosure still presents a problem in that since the absorbent layer is located afar from the point of acoustic energy generation, it is difficult to completely absorb the energy which reflects back and forth within the enclosure and to prevent the framework of the speaker unit from being vibrated. Furthermore, the conventional absorbent materials lack elasticity and viscosity so that they cannot be easily secured in position to the rearside of the loudspeaker.
In automotive applications, it is desirable to directly mount the speaker on a parcel shelf of the vehicle which serves as a baffle plate to utilize the rear compartment of the vehicle as an enclosure. However, for acoustic absorption purposes, the inside walls of the rear compartment would have to be fitted with a layer of an acoustic absorber, which would increase the labor and material costs.